Oleander Poisoning: A Preventable Illness

Oleander is also one of the most poisonous plants and contains numerous toxic compounds, many of which can be deadly to people and animals--including horses.

Photo: UC Davis Center for Equine Health Horse Report

Reprinted from The Horse Report with permission from the Center for Equine Health, School of Veterinary Medicine, University of California, Davis (UC Davis).

Oleander is an evergreen shrub that seems to grow everywhere in California—in yards, parks, and along freeways. It is often grown as a hedge that can reach up to 20 feet tall. The leaves are thick, leathery and dark green. White, pink, or yellow flowers that are sweetly scented grow in clusters at the end of each branch.

Oleander is also one of the most poisonous plants and contains numerous toxic compounds, many of which can be deadly to people and animals. It is especially dangerous to horses; signs of a poisoned horse include severe diarrhea, colic, and abnormal heartbeat.

The primary toxins in oleander are cardiac glycosides, which affect the heart. Cardiac reactions consist of an irregular heart rate, sometimes characterized by a racing heart that subsequently slows to below normal further along in the reaction. The heart might also beat erratically with no sign of a specific rhythm. Other toxic effects include nausea, excess salivation, abdominal pain, diarrhea (sometimes with blood), kidney failure, and colic in horses. Oleander poisonings can also affect the central nervous system and cause drowsiness, tremors, seizures, collapse, and even coma that can lead to death.

Several years ago, a sick 4-year-old Standardbred racehorse was brought to UC Davis' William R. Pritchard Veterinary Medical Teaching Hospital (VMTH). The owner reported that it had stopped eating the day before and was clearly unwell. The horse appeared to be in shock, judging from the color of its mucous membranes, a heartbeat racing at 160 beats per minute (bpm, a normal heartbeat for a horse is 28 to 44 bpm), and a slightly elevated temperature. It also had significant discomfort from ileus—a condition in which the bowel does not move the contents at normal rates of flow because of lack of neuromuscular control. The ileus had caused a backward flow of fluid and intestinal contents back into the stomach. Since horses cannot vomit, this poses a serious problem. To treat this condition, a nasogastric tube was inserted into the horse to drain the accumulating fluid while tests were performed to determine the underlying problem.

An electrocardiogram (ECG) revealed that the horse had ventricular tachycardia, which is an irregular and overly rapid heartbeat. Pleural effusion—fluid in the chest around the lungs—and pericardial effusion—fluid around the heart—were signs that the horse’s heart was failing. This condition was treated as an emergency with lidocaine administered intravenously to slow the heart rate. Eventually the heart rate was brought down to 60 bpm, substantially closer to the normal rate of 40 bpm than before, and the arrhythmia was converted to a normal sinus rhythm.

Blood work and urinalysis results then indicated a build-up of toxins and renal failure. By now, the horse was quite weak and was staggering. It was immediately put on intravenous fluids to flush out the toxins and eventually was stabilized. Meanwhile, testing continued to determine the exact cause of illness. The diagnosis of toxicity was confirmed by laboratory tests, which showed the presence of oleander in the blood, feces, and stomach fluid. The owners of the horse had not realized that the pasture the horse had been turned out in days before was surrounded by oleander.

Fortunately, this story has a happy ending. With continued intensive treatment and supportive care, the horse began to recover and was eventually released from the VMTH. Three months after this incident, the owner reported that the horse was doing very well and was back in training.

Not every case ends this well, so remember that preventing exposure to oleander is by far the best course of action for your horses and other animals.

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